Wood pulp and process for producing same



Jan. 15, 1963 H. L. CROSBY ETAL 3,073,737

woon PULP AND PROCESS FOR PRODUCING SAME Filed Oct. 8, 1958 2 Sheets-Sheet 1 INVENTORQ #araze/i L. Crosby A/oeZ M 6' 6 r lwdc l k ATTORNEY Jan. 15, 1963 H. 1.. CROSBY ETAL 3,073,737

WOOD PULP AND PROCESS FOR PRODUCING SAME Filed Oct. 8, 1958 2 Sheets-Sheet 2 I H W2? Mama-e W ATTORNEY Conn, assignors to Don-Oliver Incorporated, Stamford,

(302121., a corporation of Delaware Filed Oct. 8, 1958, Ser. No. 766,920 9 Claims. (CL, 1d2-'1$) This invention relates generally to the field of paper making and in particular to improved ways and means for the production of a pulpfrom wood materials that is at least equivalent in all respects to the well known groundwood pulp such as is commonly used in the manufacture of magazine paper, newsprint, molded pulp products, chipboard and the like.

Groundwood pulp, as its name implies, is produced mechanically by the application of simple brute force in an attempt to break the cellulosic structure down into individual fibers,; but whichactually results in a'pulp mainly comprised of fiber bundles. Although groundwood pulps have, and still do, enjoy wide use, they suffer from economic disadvantages arising primarily out of an increasing shortage of so-called soft or coniferous woods. As the supply of softwoods diminished the cost of groundwood pulp increased until it is now reaching an uneconomic level. In an attempt to offset rising costs and find a suitable substitute for groundwood pulp, the so-called semicliemical processes have been developed. Such processes are aimed at utilization of the so-called hardwoods or deciduous woods. In such processes, as heretofore practiced, Wood chips are treated with a solution chemical such as sodium hydroxide, sodium sulfite', sodium sulfide and the like, the goal being to have the chips penetrated by chemicals and'the lignin or lig'nin cellulosic content softened and broken down by heat and/ or chemical reaction. note arenon-uniformly penetrated and reacted, are then subjected to mechanical action whereby the fibers are more or less separated. The resulting pulp mass is then washed to remove dissolved compounds, Processes of this nature are generally known in present practice as cold soda, neutral sulfite or hardwood kraft processes.

Although such processes have found wide acceptance they do suffer from certain disadvantages chief among which is the extremely low yield due to removal of dissolved compounds during treatment. In fact, in current practice, most semi chernical processes do not give a yield above 65%. The cold soda process is an exception to this and does give a yield up to 90%, however, the cost of Thethus treated chips, which it is important to chemicals and power requirements do keep the cost of pulp produced thereby relatively high. As a consequence, despite the advantage of being able to producepulp from hardwoods, the cost is still uneconomically high due to the low yield as well as the relatively high cost of treatment chemicals and power requirements.

One of the major obstacles in prior semi-chemical process'es, especially the cold soda process, resides in the inability of the chemical solution to penetrate the chips evenly. That is to say, no matter how long the chips are soaked in the solution, there isa gradient of chemical concentration between chip surface and center. As a result, the cellulosic materials are not uniformly penetrated or reacted and it is impossible to refine them to a pulp containing fibers of uniform size and composition.

Attempts have been made to treat wood chips with less expensive chemicals such as hydroxides of the alkaline earths, magnesium, barium or calcium. But such attempts heretofore have met with little success because such chemicals are substantially insoluble consequently will not penetrate wood chips to the degree required for softening inter-fiber bonds suficiently for ready separation.

3,073,757 Patented Jan. 15, 1953 ice The present invention is predicated upon the discovery that inexpensive chemicals such as the alkaline earth hyvide ways and means for producing such pulp at a yield in the region of from %l00%.-

It is a still further object to produce such pulp at a much lower cost and from available, hence less expensive, hardwoods, such as beech, birch, maple,- oak and'gum.

Still another object is to provide a pulping process which utilizes a-much less expensivechemical a'nd at the same time enables a much more efficient use thereof.

v A still further object is to reduce or eliminate waste and recovery problems attendant upon most prior pulp ing processes. A related object is to produce a pulp at a greatly reduce'd power consumption as compared to prior processes for producing groundwood' and semi-chemical pulps.

Another important object is to provide an apparatus arrangement for practicing the process of I the invention enabling drastically reduced capital investment and operating costs.

Other and more specific objects will appear to those skilled in the art as this specification proceeds.

As noted above, prior attempts re roduce pulp with inexpensive insoluble chemicals (i.e. calcium hydroxide) have resulted in failure primarily because insolubility of such chemicals limits penetration of the wcod, hence there is-insufiicientsoftening of inter-fiber bonds to permitrelease of individualfibers. However, as also men tioned above, thepresent invention ispredicated-upon the discovery that a particular mechanical treatment of the wood in the presence of such insoluble chemicals enables suflicient uniform penetration to permit separation of fibers by the'applic'ation of minimum force. Thus, it can be seen that the manner of impregnating the chips with treatment chemicals is a critical part of the present invention. 7 g I g In connection with impregnation, we have discovered that by alternately compressing and decompressing the wood chips in the presence of the insoluble chemical a pumping-like action occurs which effects a uniform enetration of chemicals into the chips and simultaneously exerts a gentle mechanical loosening action on fibers. The chemicals react with the "inter-fiber bonding" material inthe' chips to form an insoluble reaction product.

This reaction product is utiliied in the present process a of an axially aligned smaller cylinder adjacent the inner wall. of the outer cylinder in such'a manner that material fed into the interior of the outer cylinderand allowed to rotate therewith passes repeatedly between the nip atthe point of closest proximity of the cylinders. Suitable means are provided for axially propelling the material so that it travels in a more or less spiral path and in so products, thus'siguificantly reducing doing passes repeatedly through the nip. Machines of this type have been on the market for general grinding purposes for some time and a iypical description thereof is to be found in United States Patent No. 2,674,162 issued on April 6, 1954, to A. J. Haug.

In connection with impregnation by compression and decompression, it is important that the chips not be subjected to too vigorous an action as it is desired to minimize fiber degradation during impregnation because degradation of fibers causes a breakdown into fine material which ultimately results in a loss of yield or strength or both.

Of equal importance to impregnation is the manner in which impregnated chips are refined or defibered in ac cordance with the invention. In this connection, it is important to note that the usual prior method of defibering wood chips, such as employed in semi-chemical processes, requires the pulp to be ground continuously until a minimum of fiber bundles are found. In contrast to this, the present invention is based on the premise that controlled, considerably lighter mechanical action releases, at an important reduction in power consumption per ton of product, much higher quality fibers and forms a minimum of debris.

In accordance with the present invention, impregnated chips, which are uniformly reacted with a controlled minimum of chemical, are subjected to a gentle refining at a relatively low temperature and at relatively high density.

Since only light mechanical action is used on impregnated chips, the chips are not completely defibered in a single pass but do release from their surfaces those fibers which are most easily brushed off by virtue of having been loosened by the chemical reaction occurring during impregnation.

The resulting partially refined pulp is then subjected to a screening operation by which released fibers are separated as product and reject or oversize pieces are recycled again to process for a second impregnation and further refining. This simple yet novel method of operation results in product pulp consisting almost entirely of usable fiber material. Moreover, since mechanical force is applied only to releasing fibers and not to the useless retreatment of already released fibers, a considerable power saving, in the neighborhood of 40%, is relized. Another factor to remember is that products of reaction between interfiber bonding material and treatment chemi' cals are so gently handled in the refining step that they remain suitable for use as part of the product pulp.

Operation of the refiner should be carefully controlled to separate substantially intact those fibers which have been loosened by chemical action, but at the same time observing the limitation that neither fibers nor insoluble chemical reactive products are to be broken down into debris. Also, refining should be so conducted that temperature rise during the operation causes neither formation of debris from thermoplastic compounds nor irreversible discoloration of the pulp. In general, control of the refining operation is best exercised on the basis of debris formed or, stated another way, on the yield of usable pulp. Practically, this is attained by regulating the refiner, as to speed operating clearances and pulp density, so that the eventual product pulp preponderates in individual fibers as indicated by microscopic examination, and has a freeness in the range of 400700 (Canadian Standard). Under the above conditions fiber debris will be at such a minimum that pulp yield exceeds 90% on the basis of bone dry wood initially supplied to process.

As noted, pulp prepared in accordance with this invention has a remarkably high freeness in the range of 400-700 (Canadian Standard), but more frequently in the freeness range of 500-600. As a result of such high freeness, which is due to the preponderance of free individual fibers, strength development by beating in a separate processing step is rendered much more efficient because heating is performed on fibers alone rather than on a mixture of fibers, fiber bundles and chips. Increased beating efliciency also arises from the important fact that the present invention preserves the hemicellulose which renders the pulp more readily beatable. It should be noted that preservation of hemicellulose is attained by controlling the pH to be at all times below about 12.

In connection with impregnation of the wood chips, the liquor employed is advantageously made up from a calcium hydroxide slurry to which has been added a small amount of alkali preferably in the form of sodium carbonate and in an amount sufiicient so that the ratio of calcium hydroxide to sodium carbonate is such that the pH of the resulting treatment chemical lies in the range of from 10 to 12. This is above the pH of calcium hydroxide alone which is about 9.3. Although not necessary in all cases the use of some alkali to attain a higher pH does possess the advantage of causing a slight swelling of the wood chips which seems to make the impregnation and separation easier. However, pH values above about 12, such as used in the cold soda process, are to be avoided because in this region the degradation of alpha-cellulose and solubility of hemicellulose is excessive.

The relative quantity of treatment chemicals added must be sufficient to maintain the reaction at a pH of at least 9.3. This means that there must be present at all times an excess of lime (or other alkaline earth) alkalinity. Although the exact function of lime is not completely understood, it may be theorized that it operates in conjunction with free alkali in the treatment chemicals to eventually form color stable calcium salts. The reactions involved may be visualized as (1) neutralization of the woods acids by free alkali (Na CO followed by (2) recausticizing of the resulting salts by lime to form the color stable calcium salts and regenerate alkali.

In practice the quantity of Ca(OH) to be used may be approximated in accordance with the formula: percent acetyl (CH CO-) 40=pounds Ca(OH) /ton oven dry wood. The acetyl value of woods is a well known relationship which may be readily determined in accordance with the method described by Phillips in Industrial and Engineering Chemistry, Analytical Edition, vol. 6, page 321.

It is important that the chips be treated at a proper temperature. In accordance with our invention, the minimum temperature should be suflicient to soften the chips so they will yield during impregnation rather than crack and break. The maximum temperautre should be below that at which discoloration and rapid drop in yield occur. For most hardwoods the temperatures should be maintained in the range from about 40 C. to about 60 C. Raw chips fed to the process are heated, preferably by contact with the impregnating chemicals, to a temperature of about 50 C. to insure softening. Above about 60 C. discoloration occurs, probably due to the lignin content, and there is a reduction in yield due to dissolving of hemicellulose and alpha-cellulow which is subsequently lost in washing operations. This also contributes to waste disposal problems.

Consistency of chips as fed to the-impregnator is maintained at about 40% (bone dry basis by weight) and sufficient impregnating chemicals are added during the impregnation operation to insure presence of the necessary chemicals for reaction. The total quantity of primary chemical required is in the range of 7%-10% by weight calcium hydroxide to dry wood. This satisfies the chemical demand but does not provide an uneconomic excess over the chemicals necessary to react with the inter-fiber bonding agents. Since the chemicals are added in a slurry form some dilution results and impregnated chips usually report to further process at a consistency in the range of about 30%35%. It is important, as explained more fully below, that this high consistency be maintained through the refining step.

Although prior processes required a relatively long de- With respect to refining, it'has been discovered that a relatively slow speed operation conducted at a high chip consistency, say in the neighborhood of 30%-40%, results in an extremely 'high quality pulp. This slow and so enables maintenance of processing temperatures in the previously discussed critical range, thus avoidingformation of birdseeds which might occur at higher temperatures due to softening and agglomeration of thermoplastic lignin compounds which subsequently harden as the result of chemical condensation and polymerization. Lower treatment temperatures also reduce the rate of reaction of alkali-sensitive substances, such as hemicellulose, which if allowed to react with alkaline material at higher temperatures will result in a lower yield. In short, at higher temperatures above the specified range, chemical attack on cellulosic substances increases and this is exactly the action to be avoided.

Relatively high consistency during refining contributes to high quality in the resulting pulp and, in addition, contributes directly to a more efficient use of power because forces are transmitted in a more direct manner to the yieldable wood substance, hence relatively wide spacing between active surfaces .of the refining machine is possible and under these conditions the mechanical action on chips or fiber bundles is more positive and uniform. Thus, there is a positive yet gentler shearing or rubbing action at high consistency and an increased fiber to fiber contact as opposed to the violent and very high shear rates inherent in any high speed low consistency refining where resiliency of the mass is low and fiber to metal contact is high. In brief then, high consistency is critical to enable the necessary gentle fiber rubbing action.

In order that the exact procedures employed in this invention may be more readily understood and carried into efiect, reference is made tothe accompanying diagrammatic drawings in which:

FIGJ is a diagrammatic flowsheet showing generally a preferred mode of operation for practice of the present invention.

FIG. 2 is a cutaway partial top view, also diagrammatic, of a preferred type of impregnation apparatus having in addition a special type of refining apparatus.

FIG. 3 is a simplified end view of the apparatus of FIG. 2 taken in the plane of line 3--3 of FIG. 2, certain elements being omitted for purposes of clarity.

Referring now to FIG. 1, chips-of a desirable size, generally in the range from 78 to 1%" length, are conveyed by a suitable conveyer into a soaking tank 11, where they are contacted with a heated liquid introduced through a suitable conduit 12. As soon as the chips have been brought to proper temperature (40 C.60 C.) they are removed, via a screw 13 which may desirably be a compacting screw for purposes of dewatering chips to a desired consistency of about 40%. Chips are discharged frorn screw 13 into a suitable teed hopper and screw device 14 which transports them into an impregnating mill 16. During impregnation the chips are sprayed lightly with a slurry of treatment chemicals in an amount sufiicient to provide, in combination with the chemicals carried forward from the soaking heating station, the necessary total amount of chemical required for reaction. Obviously, if water is used to heat chips in the heating chamber, the amount of chemical-bearing slurry added during impregnation will be greater than if a treatmentchemical bearing slurry is used for heating chips in the speed refining is important because it minimizes friction end 15 of the impregnator there is mounted a refiner mechanism 15 which, as can be best seen by reference to FIG. 3, is actually a cone-shaped insert into which impregnated chips are forced by the transport screw 20 and in which they receive a gentle refining-rubbing action between the spaced apart surfaces of insert 15 and the flared end portion of the impregnator. v I

With reference; to FIGS. 2 and 3, since the type of machine under discussion is well known as to construction, many details have been omitted from the drawings. However, it ist-o be understood that any suitable drive mechanisms can be employed to drive thecylinders and screw. 7 r

Refined chips from the impregnator-refiner discharge into a suitable conveyor 18 in which they are re-pulped by addition of a suitable liquid introduced via a conduit 19 in asufiicient quantity to reduce pulp consistency to roughly /2%. The resulting low consistency pulp is then passed over a screen 21 which is designed and operated to separate product fibers from oversize fiber bundles, the latter being recycled byany suitable means such as conduit 22 to the soaking-heating tank for recycle through the system. Productmaterial is drained from screen 21 into a suitable hydrocyclone or polishing classifier 23 where heavydiscard material is removed, the ultimate pulp being transmitted, via a suitable conduit 24, to a Washer or decker 25 from which product pulp is removed as cake and filtrate or wash liquid'is recycled via conduit 19 to serve as dilution liquid in re-pulper 18. i

A suitable storage tank 25 is provided to insure an adequate supply of treatment chemicals and there may also be provided a heat exchanger 27 for the purpose of heating treatment liquid to :a temperature to insure heating of chips in tank 11.

. By constant recycle of rejects and deliberate operation to produce a large amount of such rejects 'a minimum of mechanical action on fibers is required thus insuring the I highest possible yield; The amount of fibers released, and of course the percent of recycle material, ina given pass through the refiner or refining attachment of the impregnator, will depend, inter alia, upon the nature of the chips, treatment temperature and pH of treatment. The amount of recycle may vary from 20% to 50% of initial feed.

Obviously it is not necessary that the impregnator and refiner be combined in the same machine so long as both pieces of machinery are gentle in action. It is noteworthy, however, that practice of the present process docs permit use of a combined impregnator and refiner since no particular detention is required after impregnation which itself occurs in an extremely short period.

EXAMPLE I (40% birch (40%) and maple (20%), containing about 35% moisture, were used. The wood was in the form of chips in the range from A3" to 1%? long and to A" thick. The dry weight of the wood was approxi mately 30 lbs. (DWS).

The chips were immersed for 20. minutes at 46 C. and a pH of about 11.7 in a treatment liquid comprising liters water, 730 gms. Na CO and 7,000 gr'ns. Oa( OH) After soaking, which served the primary purpose of heating the chips, but served to allow absorption of a portion of the treating liquor, the chips, nowat a consistency of 45% (DWS), were fed to a type apparatus shown diagrammatically in FIGS. 2 and 3. I

The actual imp-regnator used was a machine manufactored by Anton I. Hauge of Nashua, N.H.,- under the trademark name Kollermill. During compression-decompression action in such machine l0 liters of additional chemicalof the same composition described above were added. After impregnation the chips had a consistency of 34.2% (DWS) and were then passed immediatelyv 7 through, a refining mechanism of a type similar to that shown in FIGS. 2 and 3.

After refining, the resulting mass was diluted to a consistency of /2% and passed over a classifying screen having effective openings of 0.5 mm. The underflow from the screen was then subjected to further treatment in a conventional hydrocyclone. Oversize or rejects from both the screen and hydrocyclone, totaling 15 lbs. (DWS) were recycled to process and 13 lbs. of product pulp (DWS) were collected. The remaining 2 lbs., comprising soluble organics and fiber debris, remain in white water.

In continuous operation, all material recycled to process is admixed with new feed chips and a constant circulating load of about 50% of feed is set up. Losses from the system are confined to dissolved organic matter and fiber debris eventually removed by final washing of the product pulp and have not in repeated experimental runs exceeded 8% of total feed. Thus, yields have always been above 90% and in some cases have actually been as high as 100% on a weight basis. This can be explained by the fact that the new process retains a percentage of the treatment chemicals, in the form of reaction products, in the final product.

A comparison of washed, unbleached pulp produced in the above example with a washed unbleached cold soda pulp from a commercial pulp mill using the same wood furnish, is set forth in Table I, in which the pulp of the present invention is labelled Experimental Pulp and the comparison sample is labelled Commercial Cold Soda Pulp.

Table I Expcri- Commercial mental cold soda p p p p Canadian Standard irceness 490 335 Permanganate number-as determined by procedure outlined in Tappi Magazine September 1957 135 126 Lignin-bone dry basis, percent 20 21 Alpha cellulose, percent 59 67 Beta cellulose, percent 12 Gamma cellulose, percent.-. 9 7 Valley beater evaluation at Ireeness of 150 (Canadian Standard): 17

(a) TAPPI burst factor 18 (b) TAPPI tear factor 42 35 (c) Time to irecness, minutes 18 23 Zero span breaking lcngtlin1eters at 280 treeness (Canadian Standard) 8,500 5,900 Brightness gain-points G.E. at available 012 as NaOCl 30 22 From Table I it is readily apparent that the pulp produced by'the process of the present invention is unusual in many respects. In the first place, the high freeness is totally unexpected for a pulp in such a high yield range that also preponderates in individual fibers. Such high freeness is proof of the high percentage of individual fibers in the new pulp and evidence of the absence of fiber debris.

The lignin content of both pulps is substantially the same indicating no loss of wood substance in the new process.

0t extreme importance are the figures as to the relative cellulosic contents of the pulps. Note that beta and gamma cellulose contents in the new pulp are nearly twice that of the cold soda pulp. This is another indication of the radical differences between the new pulp and cold soda pulp. Bearing in mind that the beta and gamma fractions include the so-called hemicellulose, it is obvious that far less hemicellulose loss is experienced in the new process, and retention of the hemicellulose is an important factor in the high yield of the new process. Of at least equal importance is the beneficial effect of hemicellulose retention on the beating rate of the pulp and final strength characteristics of paper produced therefrom. The data clearly show that the new pulp beats from a much higher freeness (490) than the cold soda (335) to the test free- EXAMPLE II Two separate batches of chips identical to those of Example I were impregnated with the same treatment chemicals and in the same method of Example I. Both samples were subjected to a single pass of refining, Sample A being at high consistency and Sample B at low consistency. Comparative results are tabulated in Table II.

Table II Sample A Sample 1! Consistency during refining, percent 38. 6 5. 0 Percent acceptable product-si.ngle pass 40. 0 29. 0

refinin Frecness (Can. Std.) accepted pulp 575. 0 498.0 Beater evaluation at frceness (Can. Std.):

(a) Time (minutes) to beat to treeue s 27 22 (b) TAPPI burst factor 17 12 (c) TAPPI tear factor 48 41 The above figures demonstrate the increased yield of acceptable fiber with high consistency refining as well as improved pulp strength. Note that Sample A has a much higher freeness than Sample B and that its burst and tear factors are considerably higher after beating. The time to beat to freeness 150 is higher in the case of Sample A,'but that is to be expected in view of its higher initial freeness.

We claim:

1. A chemically assisted pulping process for producing pulp from wood chips by treatment with a relatively mild and relatively insoluble alkali in the form of calcium hydroxide and a minor portion of an additional reagent providing free sodium alkalinity, followed by mechanical defibration of the thus impregnated chips, the improvement which comprises the steps of heating such chips in a soaking bath to a temperature in the range of from about 40 C. to about 60 C. and thus above the temperature at which individual fibers become resilient and below that at which irreversible discoloration and agglomeration of contained thermoplastic compounds occur, removing treated chips from the soaking bath and increasing the consistency thereof to a range of from about 30% to about 40% dry wood substance subjecting such chips in said temperatures and consistency ranges to contact by a treatment liquor substantially containing as impregnating reagent a relatively mild and relatively insoluble alkali in the form of calcium hydroxide in excess as well as a minor portion o'f an additional reagent providing free sodium alkalinity, made up to a resulting pH of 10 to 12 with the dosage of reagent chemicals applied to be such that resulting mass of chips and reagents is maintained at a final pH of at least 9.3, subjecting said chips while in contact with said reagents and at said consistency to treatment alternatingly by compression and decompression thereby effecting partial impregnation of said chips by said reagents and consequent loosening of fibers substantially at the surface of said chips, subjecting said partially impregnated chips while still maintaining them in said temperature and consistency ranges to mechanical refining only sufiiciently strong to dislodge loosened fisaid mixture while collecting the residual chips, whereby power consumption for refining as well as fracturing of fiber material and losses of hemicellulose are minimized, and recycling the collected said residual chips for retreat ment in mixture with new chips by compression and decompression for the purpose of additional impregnation and by additional refining.

2. The process according to claim 1, wherein the residual chips are recycled into said soaking bath.

3. The method according to claim 1, wherein said compression and decompression of the chips is effected by periodically recurring pressure of a pressure roller upon said chips, moving relative to a supporting surface for said chips.

4. The process according to claim 1, wherein a portion of said reagents is provided in said soaking bath, and the balance of said reagents is provided during said compression and decompression operation.

5. The process according to claim 1, wherein said separation of the loosened reacted fibers from said resulting chips is efiected by a screening operation producing screen rejects comprising said resulting chips to be recycled for said retreatment in the amount of about 20% to about 50% j and producing screen liquor containing said detached fibers.

6. The process according to claim 1, wherein said additional reagent is sodium carbonate.

7. The process according to claim 1, wherein the total quantity of primary reagent required is in the range of 7% to 10% by weight of calcium hydroxide to drywood.

8. The process according to claim 1, wherein the wood chips are hardwood and the soaking bath includes the following reagents in the proportion of about 730 grams Na CO and about 7000 grams Ca(OH) per 46 lbs. of hardwood containing about 35% moisture in about 70 liters of Water, and the balance is provided during the compression and decompression treatment operation.

9. A Wood pulp product derived from wood chips by impregnation with a substantially mild and relatively insoluble alkali substantially in the form of calcium hydroxide with the addition of a minor portion of a reagent providing free sodium alkalinity, made up to a resulting pH of 10 to 12 with the dosage of reagent chemicals applied to be such that the resulting mass of chips and reagents is maintained at a final pH of at least 9.3, said pulp representing a yield in the order of,90% including a significant amount of the original beta and gamma cellulosic content of the wood from which the pulp is 10 produced by heating the chips in a soaking bath of liquid to a temperature in the range of about 40? C. to about C. long enough to effect softening of the chips while avoiding irreversible discolorations and agglomeration of contained thermoplastic compounds, removing said chips from said soakingbath and increasing the consistency thereof to a range of from about 30% to about 40% dry wood substance, subjecting such chips in said tem perature and consistency ranges to contact with a treatment liquor substantially containing the aforementioned reagents, subjecting said chips while in contact with said reagents at said high consistency to treatment alternatingly by compression and decompression, thereby effecting only partial impregnation of said chips by said reagents and consequent loosening of reacted fibers substantially at the surface of said chips, subjecting said partially impregnated chips while maintaining said temperature and high consistency ranges to gentle mechanical refining by rubbing action upon said chips between spaced refining surfaces only sufficiently strong to dislodge loosened reacted fibers from said chips while leaving in the resulting chips substantially all unloosened unreacted fibers, diluting the resulting mixture of residual chips and dislodged fibers to a low consistency and separating said dislodged fibers from the residual chips, whereby fracturing or fiber material as well as losses of hemicellulose are minimized,

and recycling residual chips for retreatment in mixture with new chips for the purpose of further impregnation and further refining, the resulting pulp having a freeness in a range of 400-700 measured in Canadian Standard Freeness and presenting a yield in the order of including a significant amount of the original beta and gamma cellulosic content of the Wood.

References Cited in the file of this patent UNITED STATES PATENTS 1,973,637 Lathrop et al. Sept. 11, 1934 1,987,195 Kipper Ian. 8, 1935 2,071,304 Hirschkind Feb. 16, 1937 2,169,473 Olsen Aug. '15, 1939 2,182,520 Schwabe Dec. 5, 1939 2,454,532 Water Nov. 23, 1948 2,599,571 Miller June 10, 1952 2,708,160 Aronovsky et a1. May 10, 1955 2,805,168 Bradley Sept. 3, 1957 2,947,655 Eberhardt Aug. 2, 1960 2,956,918 Snyder Oct. 18, 1960 

1. A CHEMICALLY ASSISTED PULPING PROCESS FOR PRODUCING PULP FROM WOOD CHIPS BY TREATMENT WITH A RELATIVELY MILD AND RELATIVELY INSOLUBLE ALKALI IN THE FORM OF CALCIUM HYDROXIDE AND A MINOR PORTION OF AN ADDITIONAL REAGENT PROVIDING FREE SODIUM ALKALINITY, FOLLOWED BY MECHANICAL DEFIBRATION OF THE THUS IMPREGNATED CHIPS, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF HEATING SUCH CHIPS IN A SOAKING BATH TO A TEMPERATURE IN THE RANGE OF FROM ABOUT 40* C. TO ABOUT 60* C. AND THUS ABOVE THE TEMPERATURE AT WHICH INDIVIDUAL FIBERS BECOME RESILIENT AND BELOW THAT AT WHICH IRREVERSIBLE DISCOLORATION AND AGGLOMERATION OF CONTAINED THERMOPLASTIC COMPOUNDS OCCUR, REMOVING TREATED CHIPS FROM THE SOAKING BATH AND INCREASING CONSISTENCY THEREOF TO A RANGE OF FROM ABOUT 30% TO ABOUT 40% DRY WOOD SUBSTANCE SUBJECTING SUCH CHIPS IN SAID TEMPERATURES AND CONSISTENCY RANGES TO CONTACT BY A TREATMENT LIQUOR SUBSTANTIALLY CONTAINING AS IMPREGNATING REAGENT A RELATIVELY MILD AND RELATIVELY INSOLUBLE ALKALI IN THE FORM OF CALCIUM HYDROXIDE IN EXCESS AS WELL AS A MINOR PORTION OF AN ADDITIONAL REAGENT PROVIDING FREE SODIUM ALKALINITY, MADE UP TO A RESULTING PH OF 10 TO 12 WITH THE DOSAGE OF REAGENT CHEMICALS APPLIED TO BE SUCH THAT RESULTING MASS OF CHIPS AND REAGENTS IS MAINTAINED AT A FINAL PH OF AT LEAST 9.3, SUBJECTING SAID CHIPS WHILE IN CONTACT WITH SAID REAGENTS AND AT SAID CONSISTENCY TO TREATMENT ALTERNATINGLY BY COMPRESSION AND DECOMPRESSION THEREBY EFFECTING PARTIAL IMPREGNATION OF SAID CHIPS BY SAID REAGENTS AND CONSEQUENT LOOSENING OF FIBERS SUBSTANTIALLY AT THE SURFACE OF SAID CHIPS, SUBJECTING SAID PARTIALLY IMPREGNATED CHIPS WHILE STILL MAINTAINING THEM IN SAID TEMPERATURE AND CONSISTENCY RANGES TO MECHNICAL REFINING ONLY SUFFICIENTLY STRONG TO DISLODGE LOOSENED FIBERS FROM SAID CHIPS WHILE LEAVING ON THE RESULTING CHIPS SUBSTANTIALLY ALL UNLOSENED UNREACTED FIBERS, DILUTING THE RESULTING MIXTURE OF RESIDUAL CHIPS AND LOOSENED FIBERS TO A LOW CONSISTENCY AND SEPARATING DISLODGED FIBERS FROM SAID MIXTURE WHILE COLLECTING THE RESIDUAL CHIPS, WHEREBY POWER CONSUMPTION FOR REFINING AS WELL AS FRACTURING OF FIBER MATERIAL AND LOSSES OF HEMICELLULOSE ARE MINIMIZED, AND RECYCLING THE COLLECTED SAID RESIDUAL CHIPS FOR RETREATMENT IN MIXTURE WITH NEW CHIPS BY COMPRESSION AND DECOMPRESSION FOR THE PURPOSE OF ADDITIONAL IMPREGNATION AND BY ADDITIONAL REFINING. 